The present invention relates to an adaptive array antenna for use, for example, in base stations of mobile communications which has a plurality of antenna elements grouped into subarrays that fixedly define the control range of directivity.
FIG. 1 depicts the basic configuration of a conventional adaptive array antenna disclosed, for example, in Takeo Ohgane et al., xe2x80x9cA Development of GMSK/TDMA System with CMA Adaptive Array for Land Mobile Communications,xe2x80x9d IEEE 1991, pp. 172-176. M antenna elements 111 to 11M are equally spaced, for example, by a distance d, and each have the same element directional pattern 12 of a large beam width, and they are connected to a high-frequency distributor 13; received signals via the antenna elements 111 to 11M are each distributed by the high-frequency distributor 13 to channel parts 141 to 14N, that is, the received signal via each antenna element is distributed to N. The antenna element spacing d ranges from a fraction of to several times the wavelength used.
In each channel part 14i (i=1, 2, . . . , N) the received signals from the M antenna elements distributed thereto are applied to M receivers 151 to 15M, respectively. Baseband signals from the receivers 151 to 15M are provided via level-phase regulators 161 to 16M to a baseband combiner 17, wherein they are combined into a received output; the output is branched to an adaptive signal processing part 18, then the level-phase regulators 161 to 16M are regulated to minimize an error of the received baseband signal, whereby the combined directional pattern 19 of the antenna elements 111 to 11M is adaptively controlled as shown, for example, in FIG. 1 so that the antenna gain decreases in the directions of interfering signals but increases in the direction of a desired signal. This allows the base station to perform good communications with N mobile stations over N channels. An increase in the number M of antenna elements increases the gain and enhances the interference eliminating performance. At the same time, however, the number of receivers 15 also increases and the amount of signal processing markedly increases.
With a view to solving the abovementioned problems, there is proposed in Japanese Patent Application Laid-Open No. 24702/87 an adaptive array antenna of such a configuration as depicted in FIG. 2 wherein the array antenna elements are divided into groups (subarrays) each consisting of several antenna elements, the high-frequency received signals are controlled in phase and level and then combined for each subarray and the combined signals are each distributed to the N channels. In the illustrated example, subarrays 211 to 21L are formed in groups of four antenna elements, and for each subarray, the received signals are combined by one of high-frequency signal combiners 221 to 22L. Each subarray has high-frequency level-phase regulators 231 to 234 connected to the outputs of the antenna elements, in which coefficients W1 to W4 are set to regulate the levels and phases of the received signals so that the subarrays 211 to 21L have the same antenna directional pattern 24. The outputs of the high-frequency signal combiners 221 to 22L are fed to the high-frequency distributor 13, from which they are distributed to the channels 141 to 14N. The subsequent processing is he same as in the case of FIG. 1.
In this instance, the number of receivers 151 to 15L in each channel part 14i is reduced to L, in this example, M/4, and the number of level-phase regulators 161 to 16L is also reduced to M/4, that is, the amount of hardware used is reduced; besides, the gain of the overall directivity (combined directivity) of the antenna elements 111 to 11M increases and interfering signal components are also removed sufficiently. However, the range over which the combined directivity can be controlled is limited only to the range of the subarray directional pattern 24, and hence it cannot be controlled over a wide range. That is, when the direction of the subarray directional pattern is changed as indicated by the dashed line 26 in FIG. 2, for example, by setting coefficients W5xe2x80x2 to W8xe2x80x2 in the level-phase regulators 231 to 234, respectively, the range over which the combined directional pattern 19 can be regulated by the level-phase regulators 161 to 16L is limited specifically to the range of this directional pattern 26. The range over which to track mobile stations is thus limited, but a wide angular range could be covered by such an antenna arrangement as depicted in FIG. 3. That is, a plurality of array antennas 271 to 275, each consisting of the subarrays of antenna elements in groups of M shown in FIG. 2, are installed with the subarray directional patterns of the array antennas 271 to 275 sequentially displaced a proper angle apart as indicated by beams 241 to 245 and the array antennas 271 to 275 are selectively switched to track mobile stations in any directions over such a wide range as indicated by the beams 241 to 245; by this, a wide service area could by achieved. From the practical point of view, however, it is difficult to install such a large number of antenna elements as mentioned above.
A possible solution to this problem is to decrease the number M of antenna elements used and hence enlarge the antenna spacing d. In this instance, as depicted in FIG. 2, when the width of the element directional pattern 12 is large, narrow grating lobes 28 of relatively large gains, other than the main beam 19, develop in plural directions at about the same angular intervals. In the directions of the grating lobes 28, however, the BER (Bit Error Rate) due to interfering signal components increases, making it difficult to use the antenna. On the other hand, when the directional pattern 12 is narrow as indicated by a brokenline 24 in FIG. 5, no grating lobes appear as shown in FIG. 5, but the range over which to control the combined directivity 19 is limited by the element directivity 24 and a wide range cannot be covered accordingly.
An object of the present invention is to provide an adaptive array antenna with which it is possible to offer services over a wide range without involving marked increases in the numbers of receivers and processing circuits and in the computational complexity.
The adaptive array antenna according to the present invention comprises:
a plurality of subarrays of antenna elements arranged in groups of at least two, said antenna elements each outputting a high-frequency received signal;
a plurality of high-frequency level-phase regulators for regulating the levels and phases of said high-frequency received signals from said at least two antenna elements of each of said plurality of subarrays, thereby setting the directivity of said each subarray;
a high-frequency signal combiner for combining the regulated high-frequency received signals from said plurality of high-frequency level-phase regulators corresponding to said each subarray and for outputting the combined high-frequency signal;
a receiver for converting said combined high-frequency signal from said high-frequency signal combiner corresponding to said each subarray to a baseband signal and for outputting said baseband signal;
a baseband level-phase regulator for adaptively regulating the level and phase of said baseband signal from said receiver corresponding to said each subarray;
a baseband signal combiner for combining the regulated baseband signals from said baseband level-phase regulators corresponding to said plurality of subarrays, respectively, and for outputting the combined baseband signal; and
an adaptive signal processing part whereby said baseband level-phase regulators corresponding to said plurality of subarrays, respectively, are adaptively controlled based on said combined baseband signal from said baseband signal combiner to set the combined directivity of all the antenna elements in the direction of a desired signal.